This invention relates generally to computed tomography (CT) systems, and more particularly to generating images in the computed tomography systems with reduced artifacts.
A CT system typically includes an X-ray source to project a beam of X-rays through an object, for example, a patient. The beam of X-rays projected towards the object gets attenuated after passing through the object. A detector array detects the attenuated X-ray beams from various angular positions with respect to the object and generates one or more signals. The signals are processed to obtain processed data, referred to as projection data. Further, the projection data are combined to reconstruct the object images, which represent the X-ray beams attenuation through the object. A spectral calibration can be performed on the detector elements to determine spectral response differences among the detector elements and to ensure that the reconstructed images are uniform and free of artifacts. Inaccurate determination of spectral response differences between detector elements due to, for example, beam hardening through water, soft-tissue, bone and contrast agents, and detector imperfection, results in rings and bands in images.
In a single-spectral CT system, the projection data for a given path is acquired with a single X-ray (or other radiation) spectrum. The spectral calibration is performed for each incident X-ray (or other radiation) spectrum to remove beam hardening and to correct the detector imperfection. A set of calibration coefficients are computed for the incident X-ray spectrum and a correction is expressed as a function of the calibration coefficients and the projection data from the same spectrum as, p_corr=f(c1, c2 . . . , p), where c1, c2 are the calibration coefficients and p is the projection data from the single spectrum. The spectral calibration method described above, linearizes the projection data to the length of a single basis material or to the length of basis materials having similar X-ray attenuation characteristics, for example, water and soft tissue. Hence, the spectral calibration with a single X-ray spectrum removes beam hardening artifacts due to a single material, such as water, when the incident beam is attenuated by water. In a dual-spectral CT system, the projection data for a given path is measured twice by using two X-ray spectra. Hence, a projection pair data set comprising projection data from the two X-ray spectra is obtained with the dual-spectral CT system.
Currently available methods for calibrating the dual-spectral CT system may employ the spectral calibration method of the single-spectral CT system as described above wherein calibration is performed at each of the two X-ray spectra for a single basis material. These methods therefore remove beam hardening and detector imperfection artifacts for a single basis material. However, such methods do not account for image artifacts due to other basis materials, for example, bone or other contrast agents. Therefore, there exists a need for calibrating a dual-spectral CT system, which will provide beam-hardening and detector-spectral artifact free images at more than one basis material.